As a conventional system for coding a video signal with high efficiency and recording it, there has been known an image coding apparatus disclosed in Japanese Published Patent Application No. Hei. 9-18872.
FIG. 20 is a simplified block diagram of the conventional image coding apparatus. FIGS. 21 to 23 are diagrams of characteristics for deciding a quantization step by the conventional image coding apparatus. In each of FIGS. 21 to 23, the abscissa shows the size of each image feature data, and the ordinate shows the coefficient of quantization characteristic. The larger the coefficient is, the smaller the quantization step is. That is, the number of bits to be given is increased, and the quantity of generated codes is increased.
The construction and operation will be described with reference to FIG. 20. As shown in FIG. 20, the conventional image coding apparatus comprises an average luminance level extractor 2001 for extracting the average of the luminance signal for every predetermined period; an average chrominance level extractor 2002 for extracting the averages of the color-difference signals for every predetermined period; a luminance variance extractor 2003 for extracting the variance of the luminance signal for every predetermined period, a quantization step decider 2004, and an encoder 2005.
An image signal is input to the average luminance level extractor 2001, the average chrominance level extractor 2002, the luminance variance extractor 2003, and the encoder 2005. In the average luminance level extractor 2001, the average chrominance level extractor 2002, and the luminance variance extractor 2003, the respective image feature data are extracted from the input image signal over a predetermined period, and the extracted data are input to the quantization step decider 2004. The quantization step decider 2004 decides a quantization step according to at least one of these image feature data, and outputs the result to the encoder 2005. The encoder 2005 encodes the input image signal using the quantization step decided for every predetermined period by the quantization step decider 2004, and outputs it.
The quantization step decider 2004 operates as follows. When the average luminance level extracted by the average luminance level extractor 2001 is relatively small, the decider 2004 decreases the quantization step according to the characteristics shown in FIG. 21. Between two color-difference signals Cb and Cr extracted by the average chrominance level extractor 2002, when the average level of the red-base Cr signal is larger than that of the Cb signal, the decider 2004 decreases the quantization step according to the characteristics shown in FIG. 22. When the luminance variance extracted by the luminance variance extractor 2003 is relatively small, the decider 2004 decreases the quantization step according to the characteristics shown in FIG. 23. The coefficients at the ordinates of FIGS. 21 to 23 are weights on the quantization step. The larger the coefficient is, the smaller the quantization step is.
In this way, the quantization step is decided adaptively to the human visual characteristics, followed by coding.
As described above, in the conventional image coding apparatus, the quantization step is adaptively decided to the human visual characteristics by using the rough feature data of the image signal for every predetermined period, and then the image signal is coded. Therefore, when coding is performed according to the extracted image feature data to reduce image degradation, the quantity of generated codes is increased as a whole. On the contrary, when the quantity of generated codes is reduced, image degradation occurs over the entire image displayed in the predetermined period.
The present invention is made to solve the above-described problems and has for its object to provide an image coding method, an image coding and decoding apparatus, an image coding apparatus, and an image recording and reproduction apparatus, which are adapted to the human visual characteristics on the entire image, and reduce degradation of image quality while minimizing the influence on the quantity of generated codes.